Salt bath



Patented Aug, 13, 1929.

STT

ARTHUR E. BELLIS, F BRANFORD, CONNECTICUT.

SALT BATH.

No Drawing. Originalapplication filed September 10, 1925, Serial No. 55,623. Divided and this application filed March 18, 1929. Serial No. 348,145. I

This invention relates to salt baths such as are used for the heat treatment of metals and other purposes, andconstitutes a division of my pending application Serial No. 55,623, filed September 10, 1925.

In general, the invention has for its primary object and purpose to provide asalt bath composition containing in solution therewith a scavenging agent acting as a solvent for oxides and other impurities carried into the bath with the metal or entering the bath from the crucible wall, and thereby preventing contamination of the bath and maintaining the same at all times chemically neutral to the metal.

' It is also an important object of the invention to provide such a salt bath composition containing the automatically acting scavenging ingredient which will possess maximum stability, high'heat transfer efliciency and which will provide an ideal heating curve for the metal. In addition, the new bath is easily controllable as to .the melting point of its components within an ap-. preciable temperature range, whereby its viscosity may be regulated or varied without effect upon its neutrality to the chemical composition of the metal being treated.

Generically, the invention comprehends the use in such a salt bath for the heat treatment of metals of a boron compound and an alkaline chloride.

In theheat treatment of metals by immersion in a salt bath, of the eutectic single phase or the non-eutectic multi-phase type, I have succeeded in developing a bath composition which, even after excessive heating at high temperatures, is automatically maintained entirely free of contamination by impurities such as metallic oxides. Specifically, the bath hereinafterreferred to is primarily designed and intended for use in the hardening of high speed tool steels or alloys, and which bath must remain stable and without appreciable volitilization at temperatures up to 2500 F. I have succeeded in accomplishing thisre'sult by using in solution with the bath, an alkaline salt together with a boron salt, the latter constituting a solvent'for practically all oxides (as of Na, K, Ba, Fe, etc.) which fuse with the boron salt to give borates. In a preferable embodiment of the invention, I utilize a compound boron salt preferably consisting of boron trioxide {B 0 and sodium meta borate (NaBO B O I have foundin practice that the use of such a boron compound with an alkaline chloride in either a eutectic or non-eutectic salt bath, will act automatically as a solvent for any oxides or other impurities entering the bath solution and maintain the original single or multiphase character of the bath solution.

I preferably employ as an alkaline salt, potassmmphloride, though if desired, p0 tasslum nitrate or any other alkaline salt may be substituted.

In practice I have found that a salt bath having the above characteristics, may be successively used in the heat treatment of high speed tool steels or alloys, and in contra-distinction to a bath of commercial b'orax, will not deleteriously or harmfully effect the metal, as by pitting the metal surface or decarburization, of the metal. harmful effects are usually caused by thepresence of undissolved metallic oxides in the bath solution. These oxides also very seriously affect the uniformity in the rate of heating of the metal piecetas well as reacting chemically therewith to cause decarburization. By my new bath employing the boron compound as an automati cally acting solvent for such oxides, a clear homogeneous solution is maintained at all times and the viscosity of the solution remains constant so that the rate of heat trans-- Such a fer through the solution to the metal immersed therein is invariable. This rateof heat transfer may however, be 'easlly controlled and regulated by either increasing or decreasing the proportion of the borates 1n the bath which changes its viscosity, and

thus enables the metal to be treated through a desired temperature range. From the above, it will be apparent that my new bath solution provides a metal heat treating medium whereby uniformly successful results may be attained in the heat treatment of high speed tool steels or alloys owing to the entire absence of undissolved metallic 0x ides in the bath. Therefore, the bath is maintained in a stable condition and its tendency to decompose or undergo changes in composition due to chemical reaction with such oxides is eliminated. Also the 'protective action of the bath film adhering to the surfaces of the metal when 'it is withdrawn from the solution is greatly improved. J i,

I preferably use in connection withthe high temperature bath above described, a quenching bath which consists of a eutectic mixture of potassium chloride and sodium carbonate which I have found to be particularly applicable and desirable in the heat treatment of high carbon content high speed steel or alloy.

It will be understood that the relative proportions of the two boron constituents of the potassium salt may be varied in order to secure the desired viscosity of the bath solution. As an example however, of a specific composition which I have successfully used, I may refer to a bath solution containing substantially 31% of boron trim oxide, 49% of sodium meta borate, and

of potassium chloride. In this example, I have stated the proportionate quantities of the salt'components before fusion, and it is possible that in the fusion of said components, certain complex salts may be formed and that, therefore, the fused mix- "ture upon analysis, would not be found to original proportions.

contain the said components in the exact It is however, unnecess ry to consider such complex salts as may e formed in the bath solution since I have found that the characteristics of the bath solution as above referred to arenot materially affected.

I claim:

1. A stable high temperature salt bath for heat treating metals containing boron trioxide as an oxide solvent.

2. A stable high.v temperature salt bath for heat treating metals containing in solution boron trioxide and sodium meta borate as an oxide solvent.

3. A high temperature salt bath for heat treating metals consisting of a stable homogeneous solution containing boron trioxid and an alkaline chloride;

4. A high temperature salt bath for heat treating metals consisting of a stable homogeneous solution containing boron trioxide, sodium meta borate, and an alkaline chloride.

5. A high temperature salt bath for heat treating metals consisting of a stable homogeneous solution containing boron trioxide, sodium meta borate, and potassium chloride.

In testimony that I claim the foregoing as my invention, I have signed my name hereto.

ARTHUR E. BELLIS. 

